Various technologies for throttle control in an internal combustion engine equipped with a supercharger have been proposed in, for example, JP-A-2004-60479, JP-A-2001-90573, JP-A-11-351010, JP-A-11-141375 and JP-A-2000-97080. As a common feature of the conventional technologies as disclosed in these publications, a target torque or a target boost pressure is determined, and the throttle valve is controlled so as to achieve the target torque or target boost pressure. For example, in the prior art as disclosed in JP-A-2004-60479, a turbo-lag index is calculated based on the information on the boost pressure, and the throttle opening is controlled based on the turbo-lag index so as to provide desired torque characteristics. In the prior art as disclosed in JP-A-2001-90573, the throttle opening is corrected in accordance with a difference between the target boost pressure and the actual boost pressure (or estimated boost pressure).
FIG. 11 shows changes in the vehicle acceleration (B), engine speed (C), throttle flow rate (D), upstream intake air pressure and downstream intake air pressure (E) and torque (F) with respect to changes in the throttle opening (A), for comparison between an internal combustion engine equipped with a supercharger and a naturally aspirated engine having the same maximum torque. In FIG. 11, solid lines indicate the above-mentioned changes in the engine equipped with the supercharger, and broken lines indicate the above-mentioned changes in the naturally aspirated engine. As shown in FIG. 11, the vehicle having the engine equipped with the supercharger has two acceleration stages (i.e., is accelerated in two steps) due to a so-called turbo lag (or a delay in supercharging), and it takes a comparatively long time to reach the maximum acceleration. These acceleration characteristics may make the driver feel frustrated or uncomfortable.
To prevent the driver from feeling frustrated or uncomfortable during acceleration, it is desirable for the engine equipped with the supercharger to provide smooth acceleration characteristics as provided by the naturally aspirated engine. Since the acceleration characteristics of the vehicle depend upon a target value, such as a target torque or a target boost pressure, the method of setting the target value may be regarded as an extremely important technical matter to be considered for achieving ideal acceleration characteristics. However, no specific disclosure about the method of setting the target torque or target boost pressure is provided in any of the above-indicated patent publications.